Wood impregnating compositions and process



March 26, 1963 w. SCHULZ 3,083,138

WOOD IMPREGNATING COMPOSITIONS AND PROCESS Filed April 18, 1961 y In van/or WOLFGANG SGHULZ A TTORIYE'YS rate $383,138 WQGD IMP%GNATENG C(EMPGSETHQNE; AND PRGCESLS Wolfgang Schulz, Sinzheim, Kreis lluhl, Wur'ttemherg-Baden, Germany Filed Apr. 18, 19M, Ser. No. 194,2il4 Claims. ('Cl. 167-385) This invention relates to wood-preserving impregnating agents, and more particularly to improvements in the compositions and the methods of impregnation of specific wood-preserving agents which comprise mixtures of salts which are well known in the art as Wolrnan salts, i.e. woodpreserving agents which are water-soluble compositions comprising for every two gram equivalents of a salt consisting of an anion containing hexavalent chromium and a monovalent cation, 6 m gram equivalents of a fluoride salt and 11 moles of an alkali arsenate, in which composition In plus n is equal to 2.

This invention is a continuation-in-part of my applications Serial No. 465,981, filed November 1, 1954, and Serial No. 593,041, filed June 22, 1956, both now abandoned.

The above-described water-soluble Wolman salts are also known in the art as U salts if they contain chromates and fluorides, and UA salts if they contain chromates and arsenates with or without the admixture of fluorides.

It is the principal object of my invention to provide wood preserving agents which are relatively non-lixiviable from the treated wood. These agents have a predetermined composition for achieving therewith a desired optimal elfect of fixation in the wood depending upon the purposes for which the latter is to be used.

It is another object of my invention to provide a woodpreserving agent composition which permits a relatively close control of the velocity of fixation in the wood.

It is further object of my invention to provide a method of impregnating wood thereby, in the course of time, the entire wood including the core is penetrated by biocidal substances.

it is still another object of my invention to improve the impregnating eflect of the known Wolman salts to permit their selective use for impregnating Wood to be used as timber for a great variety of purposes.

These compositions of water-soluble salts of chromic acids, hydrofluoric acid and/ or arsenic acid are applied in the art of preserving timber against destruction by vegetable and animal pests.

The fungicidal and/or insecticidal properties of these compositions are imparted to the wood over a longer period through the fact that the water-soluble components of these compositions react with the incrusting and other substances contained in the wood to form compounds of low solubility.

These reactions which take place in the wood comprise the reduction of hexavalent chromium to trivalent chromium and the formation of diificultly soluble or insoluble compounds of trivalent chromium with arsenic and/ or fluorine, i.e. chromium cryolites and chromium (ill) arsenates. They are generally called the fixation of fluorides and arsenates or the fixation of Wolman salts in the wood. It is, therefore, a typical feature of this fixation of Wolman salts that chromium participates in the fixation of both the fluorides and the arsenates.

It is generally accepted in the art that these fixation processes in wood are not complete, i.e. that a lagre percentage of the impregnating salts composition is lixiviated after treatment, for instance by immersion of the wood in water, by rain etc.

The degree of actually fixed amounts of these salts in the Wood resulting from the known impregnation compositions, is subject to great fluctuations which were hitherto $83,138 Patented Mar. 26, 193

ascribed to the different natures of the woods being impregnated. For example, in an article The Invention of Wood Preserving U-Salts by Dr. Joachim E. Koch, a pioneer and significant contributor to the development of the Wolman salts, published in Chemiker-Zeitung, No. 10, May 20, 1954, it is stated that by treating sawdust with a U-salt mixture of 55 parts sodium fluoride and 35 parts potassium bichrornate, about 70% of this mixture remained in the wood after exhaustive leaching. This U- salt was called Triolith-U or Basilit-U, but when the same salts were employed on pieces of wood under actual conditions, it was discovered that only about 25% of the salt remained in the wood. The same type of experience was obtained with Thanalith-U, a mixture consisting of 25 parts sodium fluoride, 25 parts sodium arsenate and 37 parts potassium bichrornate, wherein the sawdust test showed that 90% was non-lixiviable, but in actual pieces of wood, only about 60% was non-lixiviable. It was, therefore, believed to be impossible to control generally and at will, the quantitative result as well as the velocities of these fixation processes in the wood.

It was further believed that the known compositions will not substantially corrode, if in the state of low concentration, as it is frequent-1y required in the art of impregnating wood, and that these solutions could therefore be stored, for instance, in iron containers.

It has already been proposed in the art to add a very minor amount (l2%) of preferably crystalline substances having an acid reaction, such as bisulfates, to compositions of approximately 1 mole alkali bichromate (sometimes called dichromate) and 2 moles alkali arsenate but with no fluoride. This is a UA Wolman salt free from fluorides so that in the above mentioned general formula of 6 m gram equivalents of a fluoride and 11 moles of an alkali arsenate, m must be zero. The pH of this composition in solution was, however, to be maintained at higher than 6. Consequently, complete precipitation of chromium (III) arsenate could not be achieved.

It is furthermore a known practice to add small quantitles of citric acid or soluble salts of citric acid to impregnating salt compositions containing fluorine, chromium and arsenic, which admixture is said to prevent the formation of sludge which often occurs when working with this kind of preserving salt solution.

It has also been proposed in the art to produce wood impregnating agents by preparing mixtures of chromate or bichromate salts with potassium or ammonium bi fluoride and/or arsenic compounds preferably with an admixture of other fungicides, insecticides, dyes and/or substances reducing the inflammability of the wood, in order to obtain salt compositions of particularly high solubility.

Therefore, it was thus far unknown in the art to prepare wood impregnating compositions which are adapted to preserve wood under very specific circumstances and uses. Finally, none of the known compositions, as a matter of fact, permitted the attainment of complete fixation. A complete fixation requires that substantially the entire amount of chromates, arsenates and/or fluorides originally applied to the Wood remains therein and cannot be removed from the wood by lixiviation with water.

Check tests have also revealed that with the known compositions it is not possible to achieve a control of the fixation velocity in the wood. Thus, even an admixture of as much as 0.5 mole sulfuric acid to a composition of 2 moles alkali arsenate and 1 mole alkali bichromate would not substantially improve the fixing effect of the composition. Obviously, then, the proposed admixture of 2% H corresponding to a molar admixture of only 0.14 mole H 80 is completely inelfective. When using bisulfates instead of sulfuric acid, the limiting concentration that must be exceeded, in order to improve the fixation noticeably, is even higher than that concentration in the case of sulfuric acid, since bisulfates have to be considered the equivalents of monobasic acids only.

Because these various modifications of the Wolman salts did not lead to significantly different results, in commercial practice, the Wolrnan salts are supplied at a slightly alkaline pH. In other words, the efiect of acid addition was considered unimportant and therefore a waste of money. As a matter of fact, the Koch article previously cited, on page 328, lines 17-27, states that during the chrome tests, it was singularly ascertained that bifluoride (e.g. NaFHF) did not influence the resistance to leaching of chrome.

In direct contrast to the teachings of the prior art, I have discovered that the addition of acid to a Wolman salt can greatly improve the resistance to lixiviation of the salts. As will be demonstrated hereinafter, the amount of acid that is added is critical, and this amount could not have been deduced by the teachings of the prior art. Whereas, a multitude of acids can be suc cessfully utilized, it is preferred to utilize a member of the group comprising hydrofluoric acid, sulfuric acid, acetic acid, alkali bifluorides, alkali bisulfates, alkali pyrosulfates, ammonium salts such as ammonium sulfate or acetate or bifluoride, and mixtures thereof.

I have discovered that, when using these Wolman salt compositions for the impregnation of Wood the quantity of monovalent cations such as potassium, sqdium and/or ammonium is related only slightly to the type of wood being impregnated. This is so because the kinds of Wood requiring impregnation contain, in nature, only very weak acids which acids, in fact, do not significantly influence the fixation processes in the wood. Of particular importance, it is essential to appreciate that whereas the prior art assumed the trivalent chromium ion to be bound in the form of insoluble fungicidal and insecticidal compounds, I have discovered that there are substantial quantities of non-lixiviable .chromic compounds formed, free of arsenic and/or fluorine, e.g. chromic hydroxide, which have no fungicidal and/or insecticidal effect. These undesirable chromic compounds are formed due to the fact that there i insuflicient acidity in the wood. n the basis of this fundamental discovery, I have-taken one further step and have succeeded in formulating the reactions taking place in the wood and have also determined the ratios ofthe components in the composition required for optimal performance in treating various woods. I

- I have thus found that, when using the specific Wolman salt-containing compositons, the velocity and extent of fixation reaches an optimum when the converted Wolman salt contains at least one acidic-substance which 'setsfree some of the hydrogen ions in the Wolman salt solutions, and which shall be referred to hereinafter, for the sake of brevity, as proton donors. These proton donors must supply as many equivalent weights of hydrogen ions (protons) as are required for thecomplete neutralization of the above mentioned monovalent cations. That is to say, the hydroxyl ions associated with the alkali ions liberated from the starting compositions by the fixation and reduction processes in the wood, must .be neutralized. On the other hand, if too much acid is added, the fixation products in the wood are dissolved.

In order to accomplish this neutralization, I have furthermore made a secondary discovery in that the weaker the acid used for neutralization, the lower the velocity of fixation; consequently, a control of the rate of fixation is possible.

I have found, as still another feature of my invention, that with neutral salts of hydrofluoric acid or arsenic acid (e.g. Na AsO being present in therequired molar ratios, in each case one should assume the reduction of only 1 mole alkali bichromate or only 1 mole alkali chromate, using respectively eight, or in the latter case, five equivalents of hydrogen ion. If, on the other hand, the arsenates are employed in the form of acid salts such as, for instance, Na I-LAsO the fixation velocity will only reach optimal valuesif the number of hydrogen-ion equivalents necessary for the reduction of hexavalent chromium is decreased by certain minimum amounts which depend, for instance, on the content of acid hydrogen in the acid arsenates, independent of the reaction proper of the acid arsenate" which may give in aqueous solution either an acid reaction or an alkaline one as is the case with Na I-IASO In order to be able to compute the hydrogen-ion equivalents required for neutralizing the alkali ions set free in the wood in consequence of the reduction of chromate and/or bichrorrrate and the fixation of arsenic and fluorine, it is necessary to know the chemical composition of the cryolites and of the chromic arsenate beingtormed during these processes.

I have discovered by experiments that the amount of acid required for the fixation process wit-h U- and UA salts is such that the formation of CrAsO, and Me CrF may be assumed. For example, a UA-salt composition comprising MeF, Me HAsO and Me Cr O shall be considered as a basis for computation. If, in order to facilitate the calculations, the symbol Hwd is used as indicating hydrogen as the reducing agent in the wood, and a monobasic acid HAc as indicating the proton donor in the process, the following formulae are obtained for the individual reaction steps:

(1 Bichrornate-reductionz' Me Cr O +8HAc+6HWd=2CrAc +2MeAc+7H O One of the two Cr+++-ions being formed will be consumed for the formation of cryolite, and the second for forming chromic arsenate. Me in the aforegoing and all the following formulae designates any kind of alkaline metal ion.

(2) Cryolite formation:

6MeF+CrAc =Me CrF 3 .MeAc

(3) Chromic arsenate formation:

Me HAsO +CrAc =CrAsO +2MeAc+HAc The sum reaction as obtained from combining Nos. 1 to 3, is expressed in Equation No. 4.

If m moles cryolite and 11 moles CrAsO are to be formed from alkali arsenate, alkali fluoride and 1 mole alkali bichromate following the reduction process in the wood, then in plus it must be equal 2, because there are two Cr+++-ions available. The amounts of reacting substances are then calculated on the basis of Equation No. 5:

(6) 6m.MeF+n.MeH AsO +2Me CrO l02n)HAc +6Hwd=m.Me CrF +n.CrAsO +(lO-2n)MeAc+8H O In this example the theoretically required amount of hydrogen ion equivalent is 10-21;.

As a further feature of my invention I have also discovered that the fixation velocity is greatest if approximately 0.75 to 1.2 times the theoretically required amount of hydrogen ion equivalent is used in the form of strong acids or their hydrogen salts for neutralizing the alkali ions being set free in the wood. In this case 1 mole of a strong acid, for instance a dibasic acid, corresponds to two hydrogen ion equivalents. It is to be decided in each case whether 0.75 or 1.2 times the theoretically required amount of hydrogen ion equivalents should be used for a certain determined combination of alkali chromates, or free chromic acid, alkali fluorides and/ or arsenates with an acid, or whether another amount of acid between the aforesaid limits is to be added in order to obtain a minimum of lixiviability. This must be decided in each case experimentally on the basis of individual lixiviation tests.

If the hydrogen ion concentration (pl-I) is increased beyond the 1.2 told amount, the reduction of bichromate or chromate will indeed take place very rapidly, but the lixiviability of the impregnated wood increases rapidly too. In the eluates from the lixiviation process chromium appears in its trivalent form.

On the other hand, if the hydrogen ion concentration decreases below the 0.75 fold amount, the quantitative degree of fixation may reach an optimum, if the acid content is not decreased too much below this value. However, the velocity of the chromate reduction and therewith the fixation process in the Wood also decreases rapidly. In this case chromium appears in the eluates of lixiviation in its hexavalent and trivalent form.

I have thus discovered that, if an optimal fixation velocity is desired, hydrogen ion concentration should not be greater than the 1.2 fold amount and not less than the 0.75 fold amount of the theoretically required quantity.

According to another feature of my invention the progress with time of the fixation process, i.e. the fixation velocity, can thus be varied Within broad limits by the following measures:

(1) According to a main feature of my invention the maximally required 0.75 to 1.2 fold amount of theoretically required hydrogen ion equivalents is added to the composition, or its solution.

When applying this step according to the invention, the reduction of Cr++++++ and the fixation of fluorides and/or arsenates will progress the slower, the weaker the acid used, i.e. the smaller its dissociation constant. It has also been found that fixation is slowed down if the ammonium salts of strong acids are used instead of the free strong acids, and if the valency o1 the acid anion is used as the basis for the computation of the hydrogen ion equivalents.

if, for instance, part of tee required amount of hydrogen ions is obtained from ammonium sulfate and the rest from a strong acid such as H 80 or its hydrogen salts, then the fixation velocity may be varied between several clays and several weeks according to the molar ratio chosen between ammonium sulfate and sulfuric acid. Apart from the ammonium salts of strong acids, any alkali compound may be used which is hydrolizable when brought into contact with water, and will form more or less strong acids, as will be the case with pyrosulfates.

(2) According to the invention, the hydrogen ion concentration is decreased below the 0.75 fold amount of theoretically required hydrogen ion equivalents. In this case the fixation process may also be quantitatively completed by using strong acids as proton donators, and lowering the hydrogen ion concentration to approximate- 1y 0.25 times the amount of theoretically required hydrogen ion equivalents. Fixation will then require considerably more time than when using the optimal hydrogen ion concentration. As can be seen from diagram I pertaining to Example I, the quantity of sulfuric acid chosen 6. in that example must exceed 0.5 mole, which is approxi mately 1 hydrogen ion equivalent, if a noticeable acceleration of the fixation of arsenic is to be achieved. Similar diagrams can be drawn up for all UA-compositions. Fixation will take place the more rapidly, the more the actual conditions approach the theoretically required hydrogen ion value.

However, the admixture of proton donor should not be reduced below 0.25 to 0.35 times the theoretically required amount, even when the proton donor is a strong or a relatively strong acid, i.e. an acid the dissociation constant of which is more than about l 10 (3) According to a further feature of my invention, I provide rapidly fixing UA-compositions which are characterized by the fact that part of the fluorine introduced into the wood is capable of diifusion, i.e. it is not fixed.

This step is based on my discovery that the fixation of arsenic takes place more rapidly and more completely than the fixation of fluorine.

It had been unsuccessfully tried in the past to use arsenic acid as the proton donor for supplying the entire amount of acid required for the fixation of UA-com positions containing fluorine. These attempts failed because, as I have found, at least part of the arsenic acid destined for neutralizing the alkali ions set free in the wood by the fixation process, combines with chromium to form chromic arsenate, while at least part or all of the fluorine remains unfixed.

In other words, within the limits of the equation In plus n=2, only n moles must consist of arsenic acid. Arsenic acid must not be used for neutralizing the alkali ions set free by the fixation in the wood because this would lead to the conversion of the entire chromium present into chromium arsenate, so that no more chromium would be available for fixing the fluorine in the form of cryolite.

If arsenic acid is used as a neutralizing agent, about of the fluorine will remain in lixiviable form while about 6% only of arsenic remain unfixed.

On the other hand, it is possible, due to the more rapid fixation of arsenic, to neutralize the excess of alkali ions set free in the wood by means of free hydrofluoric acid or by means of bifluorides, which neutralization leads to the formation of neutral fluorides (K 1 Na F or, possibly, 4)2 2)- If there are suflicient potassium and/or sodium ions present in the form of alkali chromates and/or arsenates as are required in the preserving agent for a satisfactory cryolite formation, the proton donor added to the composition may be free hydrofluoric acid.

The share of unfixed fluorine may be regulated in different ways, for instance, by adding the acid destined for neutralizing the developing alkali, partly in the form of arsenic acid or its hydrogen salts, and partly in the form of other acids.

Apart from inorganic acids or substances capable of cleaving oil" inorganic acids, organic acids may also be used if they are not too rapidly oxidized by bichromates or chromates. The results discussed above under items 1 to 3 are also relevant if the respective UA- or compositions contain other adjuvants such as dinitrophenols, dinitrocresols and the like, besides chromium, fluorine and/ or arsenic.

From an analysis of the preceding discussion it is believed that the novel wood-preserving agent of this invention can be best expressed as consisting essentially of 2/ z moles of a chromate, 6m/f moles of an alkali fluoride, 11 moles of an alkali arsenate and a(6+bc)/d moles of acid wherein z is the number of chromium atoms in 1 mole of chromate; f is the number of fluorine atoms in 1 mole of fluoride; m+n=2; a=0.25l.2; b is the number of alkali atoms being present as chromate; c is the number of H-atoms being present as anhydrous arsenate; and d is the valence of the acid.

The preceding formula for the ratio of components is more fully explained in the following remarks, and three examples, with particular emphasis directed to the meaning of the conditions m+n:2 and a=0.2 to 1.2.

The corresponding pairs of values of m, and n=2m can be freely selected. Their value determines only the extent to which the chromium content of the mixture in wood is converted to chromium cryolite (m mol) and chromium arsenate (n mol). Consequently, the selection of factors m and It depends solely on secondary factor-s, e.g. the intended use of the impregnated wood, on the corrosion properties of the impregnated Wood upon certain metals or of the 'type of impregnating method which is to be used. For the construction of silos, for example, a certain wood is preferably impregnated with a mixture which is free or almost free from arsenic. As another example, wood which is in constant contact with pure aluminum will not be impregnated with a mixture aciditied with much hydrofluoric acid because in the wood there are formed in addition to the ditliculty soluble chromium cryolites, easily soluble alkali fluorides which attack pure aluminum (with the formation of NagAlFa), especially in the presence of ammonium salts.

The acid content of the wood-preserving agent is determined by the factor a, which can also be freely selected within the specified limits. As stated previously, the acid content determines the fixing speed; consequently, the factor a is dependent on process considerations, primarily the particular method of impregnation and the type of wood being treated. For the Boucherie method, for example, rapidly fixing UA-salts cannot be used (i.e. a UA-salt whose chromium reduction in wood is completed in four days), because the Boucherizing takes a much longer time. On the other hand, woods which are subjected rapidly to severe conditions, such as in harbor constructions, are impregnated with a rapidly fixing salt. Furthermore, since the portions which are under water (posts) are attacked almost exclusively by animals and there is no danger of poisoning men, the arsenic content can be quite high in such cases.

EXAMPLE 1 The wood-preserving agent is to be used in the vacuum pressure method, for example, for impregnating woods for the construction of cooling towers. Because of the continuous water contact, the fluorine and arsenic components must be fixed as completely as possible. The Wood-preserving agent to be acidified is comprised of K Cr 0 Na-F and KH As0 and is acidified with sulfuric acid.

One mole K Cr O contains 2 chromium atoms; consequently, 2:2. According to the aforementioned formulation, there is in the wood-preserving agent -7.2 mole NaF U. The protective salt mixture has the following composition in percent by weight: Percem NaF (practically 100% 40.6

KH AsO (98%) 19.8 K cl Oq (practically 100%) 39.6

Forty grams of the unacidified wood-preserving agent are dissolved in about 500 g. water. Then, 31.68 g. sulfuric acid with a content of 50% H are added to the aqueous solution. Then distilled water is added up to 1000 g. With this standardized solution, small blocks of pine sap wood (5X2.5 l.5 cm. are impregnated ac cording to DIN 52 176. The small blocks which are cut into small bars after impregnation were stored for twenty-eight days and then leached out according to the same standardized process. In the collected leaching waters, chromium, fluorine and arsenic were determined and were computed into percent of the quantities of Cr, F or As, respectively, applied to the wood. After a fixing time of twenty-eight days, the following solubilities were determined: 6% As, 44% F, 6% Cr.

In a second solubility experiment with the same concentration of solution and type of wood, a fixing time of forty-two days was used. In order to avoid excessively rapid drying, the impregnated, cut up, small bars remained in .a closed container for four weeks and then were dried in the air for fourteen more days.

After a fixing time of forty-two days, the following solubilities were determined: 5% As, 41% F, 7% Cr.

As can be seen, this result is practically identical with the result obtained after a fixing time of four weeks.

, By virtue of this example, it is obvious that solutions of conventional unacidified UA-salts can subsequently be provided with sufficient acid to increase the fixation of chromium arsenate and chromium cryolite with the substantial avoidance of chromium hydroxide formation. It is more convenient, of course, if the protective salt mixtures, as such, are suitably acidified and composed at the outset, which will be shown in the following Examples 2. and 3. These examples further show that the fluoride fixing time can be controlled to a large degree. I

EXAMPLE 2 One mole NaHF contains 2 chromium atoms. fore i=2.

The 2 chromium atoms contained in the wood-preserv- Thereing agent are desired to be changed in the wood at equal parts to chromium cryolite and chromium arsenate. Consequently m=1 and n: 1.

Accordingly, the wood-preserving agent contains:

Also, the wood-preserving agent contains:

Two moles alkali monochromate contain 4 alkali atoms; therefore, 12 :4.

One mole K HAsO contains 1 hydrogen atom; therefore, c=1.

One mole bifluoride is, according to its acid content, a monobasic acid; therefore, d=1.

The acid content is a(6+l4 1) mole monobasic acid.

For the above named use, a favorable value for a is 0.50. Consequently, the wood-preserving agent must contain 0.5(9) =4.5 moles monobasic acid. Three moles of NaHF are already present in the wood-preserving agent, and therefore, another 1.5 moles NaI-IF must be added.

The composition is as follows:

or in weight percent: Percent Na CrO NaI-IF (97%) 34.2 K HAsO (98%) 26.4

The figures added in parentheses indicate the degree of The protective salt mixture as described in Example 2 is to be produced from the same components and with the same fixing speed (with the same factor a) in such a way that the fluorine component completely changes to chromium cryolite in the wood. The wood-preserving agent is to be used for the Boucherie method. As acid component, NaI-ISO is to be present in addition to sodium bifluoride which can also be considered a monovalent acid.

Therefore, the raw materials are: Na CrO- NaHF K HAsO and NaHSO Consequently, the values for z, 1, m, n, a, b, c, and d are the same as in Example 2. The bifluoride arsenate and chromium content of the preserving agent being as follows:

The acid content is also unchanged:

moles monobasic acid. Since the wood-preserving agent already contains 3 moles monobasic acid in form of 3NaI-IF it is necessary to add 1.5 moles sodium bisulfate. Therefore, the wood-preserving agent is composed as follows:

2Na CrO 3NaHF -{-K HAsO +1.5NaHSO or in percent by weight:

10 The figures in parentheses indicate the degree of purity of the salts used.

Standardized small blocks of pine sap wood were impregnated with a 4.00% solution of the wood-preserving agent and were stored and leached out for forty-two days as described under Example 1. The solubilities were as follows: 8% Cr, 7% As, 39% F.

A comparison between the Examples 2 and 3 shows that the solubility of chromium and arsenic remained practicaly unchanged, whereas, the solubility of the fluoride decreased considerably. The lowest fluoride solubility which was observed in this test was around 35% F.

A further decrease in the fluoride content does not improve the fluoride solubility. The limiting values after leaching, however, increase because the fluorine content is not sufiicient to change all the chromium to chromium ioryolite. A part of the chromium then changes in the wood into chromium hydroxide which is fungicidally ineffective, and relatively soluble.

In generaLthe preservatives of this invention are utilized at strengths of about 1.5 to 10 weight percent in aqueous solutions.

To illustrate the marked superiority of the woodpreservatives of this invention as compared to prior art Wolman salts, the following test results are presented. In these tests, the Wolman UA and the Wolman UAR compositions are tabulated as follows:

Moles: Percent by weight 5.4 Na? 30 1.1 Na HAsO 28 Nazcrzoq 0.3 Dinitrophenol 7 UAR 67 Moles: Percent by weight 3 NH F.HF 28 1KH AsO 29 1Na Cr O According to the requirements of DIN 52176, page 2, ten blocks of pine wood were completely impregnated with 4% solutions of both of the salt mixtures submitted for testing. The degree of saturation was determined by analyzing the wood blocks before and after saturation. Immediately after saturation the blocks were separated into small sticks of about 3 mm. thickness. They were then loosely put into a glass container. This glass container remained covered for fourteen days with a glass plate and was then kept for fourteen more days while being opened ed and on, and at the end was completely opened.

Then came the leaching. The little sticks of the ten blocks of each group were covered with 600 cc. of distilled water in a glass vessel and saturated. After one hour the liquid was poured off. The flask then remained standing with 600 cc. of distilled water, which was constantly renewed, for two hours, four hon-rs the first day, eight hours the second day, eight hours the third day and seventy-two hours the fourth to the sixth day. Inbetween the wood blocks remained in closed containers without water.

The salt mixtures themselves and the leaching water were examined as to their content of fluorine and arsenic. Inthe fluorinedetermination, distillation was carried out and titration took place with thorium nitrate (Strache, R: Holzforschung, 7, 1953, pages 111 to 116, and Berghoff, W. H., 11, 1957, pages to and in the arsenic determination, the operation was carried out according to the distillation method for S-valence arsenic (E) Fischer Method, modified by M. Rohmer: BDC, GES., 34, 1901, pages 33 and 1565.

The chemical analysis of the leaching water are listed in Tables I and II for the fluorine and arsenic quantities leached out of the Wood.

Table LEACEED our FLUOR'INE QUANTITIES Leaching out leached out fluorine quantities 1 Duration W'olman Wolman Sequence in hours UAR" 67, UA percent percent 10.8 54. 2 6. 3 1. 7 5. 9 ll. 9 7.6 8.4 4. 2 4. 3 59 4. s

1 Flourlne quantities expressed in percent of the fluorine absorbed by the wood upon saturation (impregnation).

I This result means that practically all fluorine has been leached out. In this connection the amounts above 100% are merely within the range of experimental error.

i! Arsenic quantities expressed in percent of the arsenic absorbed by 40 the wood upori impregnation (saturation).

Results-The chemical examination of the leached waters according to DIN 52176; page 2 has shown thatthe resistance of wood impregnated with fluorine and arsenic is considerably greater with Wolman UAR 67 as compared to Wolman UA.

Further tests were conducted to determine the toxicities of the salts with respect to insect larvae.

The examination was carried out according to DIN 52165. Pine blocks (dimensions 5 cm. by 2.5 cm. by

10 1.5 cm.) were impregnated completely with different dilutions of Wolman UAR .67 and Wolman UA, whereby as solvent, distilled water was used. Subsequently, a portion of the little blocks was kept for four weeks at 20 C. and 75% relative humidity. The other portion was subjected to a so-called humid storage'whcreby the blocks were kept for two weeks in a container which was first closed, and slowly opened during the following two weeks. 7

Some of the blocks were subsequently subjected to a complete leaching out according to DIN 52176, page 2.

This was a so-called stationaryleaching out with two intermittent drying periods weekly. At the beginning of leaching out and after each drying period, the blocks were completely saturated with distilled water. The water was changed twice daily. The entire duration of leaching took four weeks. Thereafter into suitable holes in each little block ten egg larvae of the Hylotrtipes bqjulus L. were inserted. The animal experiments were carried out at 20 C. and 75 relative humidity. After four and twelve weeks the state of the larvae was determined after splitting the little blocks.

As the limit of toxicity of the salts, that concentration is given at which all animals were dead, or the next lower step where not all animals were dead. These two figures,

in kg. to m? wood, are the toxic values of the salt solutions with respect to the larvae and time.

For control of the test conditions the untreated little blocks were used also in the same manner.

Results are shown in Tables III to VI.

the given testing Table 111 Results otthe testing ol toxic etlect 0t Wolman UAR 67 of "Hausbock-egg larvae according to DIN 52165 Condition of the larvae in 7 percent oi the stated Duration Content of Absorbed animal test subjection of the Drying of the of the Wolrnan quantity treated little treated little animal UAR 67 in of Wolman blocks blocks tests in the impregna- UAR 67 in Dead weeks tion solution kg. per mfl I in percent woo Living Not in- Inserted serted 2 13 0 100 O 1 ti. 5 O 100 0 1 4 0. 4 2.5 0 100 0 a s e; 0.1 l drymg- 0.1 0.61 o 100 o 0. O4 0. 24 [l 70 30 12 0. 02 0.12 0 20 SO 7 t 0. (ll 0. 059 0 4g 18% Without leaching. Untrcaiezd WW {5 8 1% 0 1 o. 4 o 100 o 4 0. 4 2. 5 Q 100 0 0.2 1. 2 8 mg 1 2(0) 0.2 1.2 V Humid storage" 01 61 0 100 0 0.04 0.24 0 100 0 12 0.02 0.12 0 70 30 0. (l1 0. 060 0 60 Untreated comparison wood 0 0 1Q!) 2 12 0 100 O 4 1 6.2 O 100 0 o. 4 2. 5 0 8 28 I le v Humidstora e 0.4 2.5 0 with ashm" g 0. 2 1. 2 o 100 o 12 I 0.1 0.62 0 40 0.04 (X24 0 60 40v I (It-O2 0.12 a 0 40 60 Untreeted comparis onwood 0 v0 1 The Wood was notsplit to the end.-

Table IV Results of the testing of toxic effect of Wolman UA of Hausbook-egg larvae according to DIN 52165 Condition of the larvae in percent of the stated Duration Content of Absorbed animal test subjection of the Drying of the of the Wolman quantity treated little treated little animal UA of Wolman blocks blocks tests in intheimpregna- UA in Dead Weeks tion solution kg. per mfi in percent wood Living Not in- Inserted sorted 2 13 100 0 1 6.5 0 100 0 4 0.4 2v 6 0 100 0 3- s a as 1 drymg 0.1 0.60 0 100 0 0. 04 0.24 0 S0 12 0. 02 (1)2 8 38 1'60 0.01 0 Without leaching"- Untreated comparison wood 0 0 100 2 13 0 100 0 4 1 6. 4 0 100 0 0.4 8 108 1 8 a 0. 2 2 Humid storage 2 L 3 0 100 0 0.1 0. 60 0 100 0 12 0.04 0.24 0 0 100 0. O2 0. 12 0 10 90 0.01 0. 061 0 20 Untreated comparison wood 0 O O 2 14 0 0 1 20 4 2 14 0 9O 10 With leaching Humid storage 4 8 0.2 1. 4 0 5O 50 12 0. 1 0. 65 0 0 100 O. 04 0.25 0 0 100 0. 02 0.13 0 10 Untreated comparison wood 0 0 1 The wood was not split to the end.

Table V TOXIC VALUES OF WOLMAN UAR 67 WITH RESPECT TO HAUS- BGCK-EGG LARVAE ACCORDING TO DIN 52165 Toxic values Duration Duration of storage of the subjection of the Drying of the of the animal In percent In kg. protreated little blocks treatedlittle blocks treated tests in content of teetive wood in weeks impregnameans per weeks tion solum5 wood tion 4 0 1 0.2 (Mil-1.2 (17mg 19 12 0 04-01 0.24-0.61 Without leachmg. 4 0 2 4 L 2 5 Emma 19 i 12 0 02-0. 04 0.12-0.24 With leaching Humid storage. 19 jig 5: gig;

Table VI LARVAE ACCORDING TO DIN 52165 Toxic values Duration Duration of storage of the subjection of the Drying of the of the animal 111 percent In kg. protreated little blocks treatediittle blocks treated tests in content of tective wood in weeks impregnomeans per Weeks tion solu- In. wood tion 4 0.1 0. 2 0.60-1.13 v. drmg 19 12 0. 04-0. 1 0. 24- 0. e0 W1thoutleach1ng 4 0 2 4 1 3 8 Emma 19 12 0: 040.' 1 0: 24-0160 With leaching Humid storago 21 fi To summarize the highly significant results of these tables:

Table I shows that the fluoride component of the 01d UA-sa1t has been completely washed out, whereas the fluoride component of UAR 67 was only washed out by 43.7%.

Table II shows that the arsenic component of Wolman UA was washed out by 28.4%, whereas the arsenic component could only 'be washed out by 3.6%, which 75 data proves the improvement in fixation by this invention. The last column of Table V shows that pine splint wood which was impregnated with 0.62 to 1.2 kg. UAR

0 67 m. was sufficiently protected against attacks of larvae after complete, standard washing.

Table VI (last horizontal column) shows that 14 kgJm. of the old UA-salt must be used to protect the wood to the same extent against larvae after equal, standard washing. It has thus been shown how to increase if the composition or its solution contains, apart from the amount of acid required by this invention, at least one sodium ion per chromium atom as well as at least one, and, at will, more potassium ions.

This indicates that different types of cryolites are being formed, and further that the lixiviability of chromium and fluorine from woods which have been impregnated with salt compositions of the aforesaid kind is largely dependent, in spite of a complete reduction of bichromate, on the type of alkali ions chosen for these compositions.

If, for instance, pine wood is impregnated with a composition consisting of 7 moles potassium bifluoride, 1 mole alkali bichromate, and 1 mole potassium bisulfate, the lixiviability of chromium and fluorine in this case is, to a high degree, dependent upon whether chromium is used in the form of sodium bichromate or potassium bichromate. This is the more surprising since no essential differences can exist between the progress with time of the bichromate reduction, the concentration of hydrogen ions (pH) being the same in both cases.

Thus, if 20 to 70 percent of the alkali ions applied in the form of alkali bichromate or alkali monochromate, alkali fluoride and/ or alkali arsenate consists of sodium ions, the lixiviability of the wood preserving agent from the wood impregnated therewith is further decreased.

That fluorine fixation in the wood may lead to different final products, as has been observed, is further indicated by the fact that iron corrosion can be extraordinarily dependent upon the sodium content of the compositions. Obviousl coatings of different compositions are formed on the iron, which coatings resist the further attack by acids strongly in one case, but weakly in another.

At least one-tenth of the alkali ions applied in the form of alkali bichromate, or alkali monochromate, alkali fluoride and/ or alkali arsenate should consist of sodium ions in order to reduce iron corrosion.

According to a still further feature of the invention the drawback of water soluble chromium-ammonia complexes being formed, is avoided by using salt compositions containing ammonium ions exclusively in mixture with sodium and potassium ions, and not alone. The use of some ammonium ion is per se desirable as it provides for reaction in which the wood contains, after impregnation, exclusively diificultly soluble chromium cryolites and chromium arsenates, but no other t y-products which remain in the wood for a prolonged period, for the only byproducts resulting in the following equation are ammonia and water. 4KF.HE+2NH F.HF+Na Cr O +6H:

2K NlaCrF+2NH5+7H O Thereby the electric conductivity of the impregnated wood remains low or is greatly reduced over that of woods impregnated with compositions which leave as by-products easily soluble salts.

As yet another feature of the invention, wood is'impregnated down to the center of the core by using specific compositions containing a certain amount of soluble, nonvolatile fluorides capable of diffusion. The central part of a tree trunk, ie. the core or xylem portion cannot be impregna ed in he lt y co d o b y subsequently be attacked by wood destroying fungi. Among these are the lentinus sqamosus and other lentinus species. Wood attacked by these'fungi will become moist and water permeable- The core Wood or xylem can be impregnated by a method according to the invention and with a specific salt composition which contains fluoric acid or alkali bifluorides which entirely or partially neutralize the monovalent cations set free in the wood and form neutral fluorides therewith. Since the fixation of arsenic is much more rapid than the fixation of chromium and fluorine, a protective layer of diflicultly soluble arsenates is formed in the outer strata of the wood, while the water soluble neutral fluorides remain inside this layer protected against lixiviation. As, in the course of time, the core of the wood becomes moist, these fluorides are capable of diffusing into the core or xylem and prevent growth of the above mentioned fungi therein.

Obviously, it would be of no advantage to introduce rapidly fixing salts into the wood by means of those known methods which rely, at the same time, on the diffusion of water-soluble salts. For instance, it can frequently be observed that the sap-conducting vessels of the wood (phloem) become gradually obstructed in particular by the very voluminous chromic arsenate during a process of sap displacement such as the Boucher method. In this case it would be more advantageous to compose the salt mixture or its solution in such a way that fixation does not take place too rapidly. On the other hand, the use for which the impregnated wood is destined often requires a fixation of the introduced preserving agents which is rapid and as complete as possible.

The lixiviability of UA- and U-salts is conventionally determined on the basis of standard tests one of which may be carried out as follows:

Small cubes of pine sap wood of predetermined dimensions are dried at 220 F. and We hed a te 9 1.- ing. Then they are saturated in vacuo with the solution to be tested, then dried externally and weighed again. The absorption of solution and/or preserving agent is computed from the weight difference. The cubes are then split into small sticks and are stored for a fortnight, in a covered vessel, then air-dried for another fortnight,

and finally lixiviated. Lixiviation is carried out by quantities of 60 cc. Water per original cube, the sticks being saturated therewith in vacuo. The water is renewed from time to time. Finally approximately 360 cc. will have been used per cube. The whole process of lixiviation lasts for seven days. The optimal values for lixiviation can thus only be obtained within the 28 days required for storing in the aforesaid standard test, if fixation by 'the U- or UA-salt in question has been completed within this time.

According to this standard test, it would correspond ingly be sufiicient to provide U- and UA-salts requiring a fixation period of less than 28 days. In practice, however, the preservation of wood has been improved in many cases if U- and UA-salts of a very slow fixation velocity are used instead of very rapidly fixing salts. This is, for instance, the case when im pregnating, in particular, pine or fir Wood by the vacuum pressure process, because this treatment usually leads to a low depth of penetration. After the impregnation proper, the preserving salts must be distributed through the wood by way of ifins m. how ver. h i net guaranteed when exclusively using rapidly fixing salts. The above described working procedure therefore comprises also preserving salt compositions of long fixation periods.

The following explanation will show in greater detail the various fields of wood treatment to which the compositions according to the invention can be applied.

Timber, which is to be used for underwater constructions, and in particular for port constructions, i.e. for constant submersion in sea water, must be treated with agents containing preferably arsenic and chromium compounds only or at least a large excess of arsenic over any fluorides present, because the fluorides are convcrted by the calcium compounds present in the sea water into calcium fluoride which is, indeed, difiicultly soluble, but is, on the other hand, ineifective as a fungicide or insecticide.

On the other hand, the presence of fluorine compounds in the Wood is desirable in those portions of wooden structures which protrude above the water level, because the wooden portions not covered by water are, in the first place, subject to attack from wood-destroying fungi, while the wood portions below the water line are attacked by ocean fauna such as, for instance, Teredo navalis and 'Limnoria species, against which chromium (III) arsenate (CrAsO is particularly active.

In contrast to the timber to be used in underwater construction, structural parts of timber which are to be used in living quarters, or in mining pits and galleries, or as wooden ties for railroads, must not be treated with preserving agents containing arsenic, because such wood may be used, after it has served any of the aforesaid purposes, as fuel. For certain fungi such as Lencites trabea and certain molds convert the arsenates in the wood into highly poisonous, gaseous arsenic hydride or organic arsines. When burning timber containing such arsenic compounds the smoke developed from the burnt material will contain arsenic oxide. Timber destined for the aforesaid purposes is, therefore, treated with arsenic-free wood-preserving agents.

While in the two above described cases the wood-preserving agent has to be selected to meet specific requirements, which make it necessary to use agents free from fluorine or free from arsenic, it is often desirable to work with impregnating salts, which contain, apart from hexavalent chromium, both fluorine as well as arsenic. Thus, wooden beams which are to be used, for instance, as telephone masts, must be introduced partly into the ground and partly stand above the same.

Due to the fact that, especially in soils rich in lime, fluorine may be converted partially or entirely to the non-biocide calcium fluoride, it is always necessary that the portions of the wood to be embedded in the soil are impregnated with agents containing not only fluorides but also arsenic. Arsenic and fluorides are highly effective not only against wood-destroying fungi but also against insects such as termites, larvae of Hylotrupes bayulus L. and other wood-destroying species.

Since it would be too expensive to apply one type of impregnation for the upper and another type for the lower part of beams destined, for instance, as telephone masts, impregnating agents are preferred which contain both fluorine and arsenic.

In order to illustrate the invention still further, the following examples are given which are, however, not intended to limit the invention.

In all of these examples where a certain amount of an alkali metal bichromate is mentioned as present in the prepared composition, such amount can, of course, be introduced either directly in the form of bichromate or in the form of a correspondingly high amount of monochromate which amount can easily be calculated on the basis of the stoichiometric ratios between the molecular weights of the monochromate .md bichromate in question.

EXAMPLE IV A wood-preserving composition is prepared for impregnating timber which is to be used for constructions under sea water.

In applying the equation:

m is made equal to zero, i.e. It becomes 2 and the following reaction results:

18 On the. basis of this equation, which can be used according to the method of the invention, as an adequate basis for determining the components forming the desired composition, the latter is prepared adding as the proton donor fifty percent only of the required theoretical amount ofhydrogen equivalents. Therefore:

3.36 kg. Na HAsO (free from water) and 2.64 Na Cr O .2H O

are dissolved in about 100 liters of water, and 1.45 kg. crude concentrated H 50 (90%) are added thereto and the mixture diluted to a total volume of 200 liters. The resulting 3% solution of'the composition according to the invention is used to impregnate a pine trunk, freshly cut and still containing cap, bast and bark by the Boucher process known per se. Approximately 2 cubic meters of wood can be impregnated with the above-stated amount of solution. The fixation requires about 4 to 5 weeks to be complete.

EXAMPLE V In the aforesaid Equation A, given in Example IV, In is made equal 0.25 and n is, consequently 1.75. The disodium arsenate in the equation is replaced by monosodium arsenate and sodium bichromate by potassium bichromate. The equation then reads as follows:

indicates, in a conventional manner, that mixed crystals will be formed as far as the crystal structure permits.

of the theoretically required moles of proton donator are added in the form of H 50 The following composition results:

29.5 kg. HaF 133.5 kg. NaH AsO and which total of 300 kg. is dissolved in water; 92.4 kg. cruds H 80 (concentration are added thereto. The mixture is introduced into the storage container of vacuum pressure impregnating installation whose impregnation vessel has a capacity of 15 cubic meters, and the mixture in the storage container is then further diluted with water'to a volume of 10,000 liters. Thereby a 3% solution of this composition according to the invention is obtained. The impregnating vessel is then charged with white-scaled dry pine Wood, the vessel is evacuated to a vacuum of 20-30 Torr, and the solution from the storage container -is then flooded into the vessel under vacuum. After pressure has been balanced, the solution begins to penetrate into the wood. An excess pressure period may follow as in the known process. Fixation requires 10 to 14 days to be complete.

EXAMPLE VI In order to prepare a wood-preserving agent for the impregnation of timber that is to be used in building houses, the components of the Equation A in Example IV are so selected that n is equal zero, i.e. m is equal 2. The left side of Equation A then reads:

and by providing 7.moles HAc in the form of hydro fluoric acid H F and the remaining mole of proton donators in the form of NaHSO the Equation C is obtained as:

(C) 7KF.HF+5KF+Na Cr O +NaHSO +6H By omitting the unreacted moles of potassium fluoride,

19 the Equation D is obtained, which corresponds to the reactions actually taking placein the wood:

This equation also shows that Equation A can be used as the basic equation for computing allsalt compositions according to the invention and that it can be readily adapted to all specific embodiments of compositions required for a specific impregnation purpose.

A composition prepared on the basis of Equation D is the following:

113.4 kg. KRHF 61.8 kg. Na 'Cr O- 2H O 24.8 kg. NaHSO, (anhydrous) These 200 kg. of salt compositions are dissolved in water and introduced in the storage vessel of the vacuum pressure installation used in Example V. The solution in the storage vessel is then further diluted to a total volume of 10,000 liters, resulting in a 2% concentration of salts in the solution. White-scaled dry oak wood is then charged into the impregnating vessel of'the appara tus and impregnated therein in a manner known per se. The fixation period is approximately 8 days.

EXAMPLE VII A wood-preserving agent according to the invention is to beprep-ared which is particularly suited 'for impregnatmg timber to be used for general purposes, and, more particularly, for telephone or telegraph masts.

For this reason In in Equation A of Example IV is chosen equal 1, ie n is also equal 1. As an arsenic component anhydrous sodium meta-arsenate is to be used which is obtained from monosodium arsenate by dehydration, and is reconverted to the latter when being redissolved in water:

As an acid component, potassium bifluoride is used. Half the amount of the chromium-containingcomponent consists "of chromic acid.' The acid content then corresponds to 72% of the theoretically required amount of hydrogen equivalents. The following Equation E results: (E) 6KF.HF+2NaAsO +2Cr0gK Cr O +12H A composition, prepared on the basket this Equation E contains 37.3 kg. KRHF 23.3 kg. NaAsO 23.4 K2C1'307 16.0 kg. CrO present in the solution as H CIO The 100 kilograms of this composition are then gradually added to a commercial paste-forming agent, for instance to a solution of0.5 to 1 kilogram of the sodium salt of carboxymethyl cellulose in 15 liters of water, in a mixer provided with a motor-driven stirrer which is in operation during the mixing step.

The resulting paste may be further diluted with water to give it a desired consistency.

Sap-fresh pine poles are sealed white and then covered with the above paste, until 5 to 6 kg. of paste are ap plied per square meter of wood surface. The poles are then stacked closely packed and provided with a watertight cover such as bitumen cardboard. The stack is left standing for about 2-3 weeks. The salt has then deeply penetrated'into the wood. .After drying, the thus impregnated wood can be directly used for building porches, interior construction, telegraph poles .andthe like.

EXAMPLE VIII This example shows the unexpected detrimental efiect' of an excess of arsenic present in the composition. Ac-

cording to the condition m+n=2, only n moles of arsensic should be present in the composition as arsenic acid. The alkali or ammonium ions set free in the wood by the fixation process must not be neutralized with arsenic acid, for the presence of the latter would impede the fixation of the fluorides.

l have formulated these reactions in the following equations: (F) 6NaF+0.5As O +Na Cr O-,+1.25A O -t-6H The cryolite and disodium arsenate react further with each other according to G Na CrF +Na HAsO =CrAsO +SNaF-i-NaF This shows that the entire fluorine remains in soluble form, and my tests have confirmed that about of the fluorine can be removed from the wood by lixiviation after the fixation process is terminated. On the other hand, only 6% can be lixiviated, if lixiviation is carried out by the standard method described hereinbefore. A composition based on Equation F and comprising 26.4 kg. NaF 42.3 kg. As O leads therefore only to the aforesaid highly unsatisfactory results.

EXAMPLE IX This and the following Examples X and XI illustrate the neutralization of fluoric acid ions present in the wood-preserving composition either in the form of hydrofluoric acid or as bifiuorides. The resulting neutral fiuo' rides of monovalent cations [K F Na F or (NE F are protected against lixiviation from the impregnated wood by a surface layer of more rapidly and more completely precipitated (fixed) arsenic compounds, while these fluorides remain capable of diffusion into the xylene toward the core of the wood as aging of the latter proceeds.

In the reaction equation of Example VII, the resultant 1 mole of NaOH and 1 mole of KOH are successively neutralized with an alkali or ammonium bifluoride, such as for instance KF.HF. The following equation is obtained:

85% of the theoretically required acid is added to the following composition in the form of KRHF and CrO 44.3 kg. KFl-[F 20.7 kg. NaAsO}; 14.2' kg. CrO present in the solution as H CrO 20.8 Kzcrgoq The above kg. of a Wolman salt composition are dissolved in water and the solution introduced into the storage container of a vacuum pressure apparatus and diluted therein to a total volume of 10,000 liters. A 1% solution is obtained. The impregnation vessel of the apparatus is charged with saw timber of pine woodwhich is impregnated in the vessel in the manner described in Example V. It is recommended to allow for an impregnation period of 1 to 2 hours, at an excess pressure of 5 to 6 atmospheres, after the pressure difierence between storage container and vessel has been compensated. The fixation period requires from 10 to 14 days.

EXAMPLE X Still'more fluorine capable of diffusion is provided in the following equation:

21 100% of the theoretically required amount of proton donator are added in the form of KFHF and CrO The composition is made up of:

49.8 kg. KFl-IF 18.6 kg. NaAsO 12.8 kg. CrO present in the solution as H CrO 18.8 kg. K Cr O A 1% solution is prepared and used in the same manner as described in Example IX to impregnate saw timber of pine wood. The fixation period is only 6 to 10 days owing to the higher amount of acid added.

EXAMPLE XI In the same manner as described in Example IX, a 1% solution is prepared; however, the amount of proton donator added is 71% of the theoretical value. Fir plants and boards are impregnated with the resulting solution in the same manner as described in Example IX. The fixation period is increased to two to three weeks.

EXAMPLE XII Comparative lixiviation tests were made with pine sap wood blocks according to the standard testing method described in detail hereinbefore. The wood blocks were impregnated with 3% impregnating solutions of the cornpositions of Examples IX, X, and XI.

The lixiviability of chromium, arsenic, and fluorine is shown in Table 2.

In all these lixiviation tests, the pH value of the eluates was between 4.8 and 5.2 and was determined with a foilcolorimeter of Wulfi-Lautenschlager. This pH value corresponds to the natural pH value of pine sap wood determined in the eluates of non-impregnated block samples, and thus shows that all fiuoric acid contained in the impregnating composition had actually been neutralized in the wood.

EXAMPLE XIII The alkali bifluorides in the aforegoing examples may also be replaced by hydrofluoric acid, if there are present in the composition sufiicient amounts of sodium and potassium ions in the form of alkalichromates and alkaliarsemates to satisfy the requirements of cryolite formation.

If, in accordance with m+n=2, m is chosen equal 1, and n is, consequently, equal 1, and Me is chosen as potassium and sodium, Equation A reads as follows:

(J) NaF+KF+KH AsO +Na Cr O +6HP +6H =Na KCrF +CrAsO +5NaF+KF+7H O or written in abbreviated stoichiometric form:

(K) KH AsO +Na Cr O +6HF+6H =Na KCrF +CrAsO +7H O A composition is correspondingly prepared from 30.1 kg. KH AsO 49.8 kg. Na Cr O .2H O 40.2 kg. H F (50% concentration) 120.1 kg. total The above 120.1 kg. of salt mixture is used in the form of a paste for a subsequent supplementary impregnation of the particularly sensitive soil-air border zone of telephone poles already impregnated and plated in the soil.

In order to carry out this subsequent treatment a hole of about 2 feet depth is dug around the pole, the latter is cleaned with a brush from adhering soil, and about 450 grams of the paste are evenly spread over the mast surface in a zone extending from one foot below the soil surface level to 8 inches above the same. The pasted zone is covered with asphalted cardboard which is nailed on to the pole and sealed with bituminous paste about the top and bottom edges. The hole is then refilled with soil.

Lixiviation tests similar to those given in Example XII were made with a 4% solution of the above composition and with pine sap wood belocks of 5 x 2.5 x 1.5 centimeters, the 5 cm. edge being parallel to the fibers of the wood. 50-55% of the hydrofluoric acid applied to the wood had penetrated into the interior and remained therein. Furthermore, the electric conductivity of the wood decreased steadily in the course of 8 weeks.

The surprising decrease in the lixiviability of fluorine by a simultaneous presence of sodium and potassium ions in the impregnating composition is illustrated by Examples XIV, a, b, and c.

A composition is prepared which contains at least, apart from the required amount of proton donator, one gram atom of sodium for each gram atom of chromium, as well as a random number of gram atoms of potassium, but always, of course, at least one gram atom of K.

If, in Equation A and according to m +n=2, n is equal 0, m is, of course, 2. The required 8 moles of proton donator are added in the form of 1 mole of an alkali bisulfate and 7 moles hydrofluoric acid. The following general equation is obtained in which Me is either Na or K.

EXAMPLE XIV (a) In the first of a series of three comparative tests, potassium alone is used as Me in Equation L and the following composition is obtained:

22.4 g. KFHF g. K2CI'207 5.6 g. KHSQ;

The 40 grams of total composition are then dissolved in distilled Water in a polyethylene vessel and diluted further with distilled water to 1000 milliliters. The resulting 4% solution is used to impregnate standard blocks of 20.4 g. NaF.HF 14.0 g. Na Cr O ZH O 5.6 g. NaHSO (c) :In a third comparative test, the preferred composition containing both sodium and potassium salts is prepared by mixing 22.7 g. KFIIF 12.3 N32CI'207-2H2O 5.0 g. NaHSO Compositions (b) and (c) are used in the same manner as described for composition (a). The lixiviation results are compared in Table 3.

23 Table 3 LIXIVIABILITY OF CHROMIUM AND FLUORINE COMPONENTS IN PERCENT OF THE AMOUNTS CONTAINED IN THE STARTING COMPOSITION Lixlviability (in percent) Example Cr- F 845:1 88i5 865:1 855:5 17i0.5 31i2 EXAMPLE XV Similar comparative tests as in Example XIV were made to show that lixiviability of fluorine is also dras- (M) 6NH RHF+K Cr O a composition is prepared which contains:

21.5 g. NH F.I-IF and 18.5 g. K Cr O The composition is used for impregnating wood samples in the same manner as described in Example XIV. V

(b) In a second test, a mixture of ammonium and potassium bifiuoride is used replacing the fluoride component of test (:1). Furthermore, sodium instead of potassium bichromate was used.

On the basis of the resulting equation (N) 4KF.HF+2NH .HF+Na Cr O- +6I-I =2K NaCrF |-2NH OH+ 5H O a composition is prepared which contains:

. 17.2 g. KRHF 6.3 g. NH Fl-lF 16.5 g. Nagcrgoq-zHzo All other conditions of this test including the addition of proton donator are the same as under (a). The results of both tests are compared in Table 4 below, which shows the liXiviability of the fluorine and chromium components in bothcases, in percent of the amounts of these components in the compositions applied to the Wood samples for impregnating the same.

In both tests the concentration of the impregnating solution is 4%.

Table 4 Lixiviabllity (in percent) Example Cr F XV (a) 85i1 90*5 XV (b) 153:0. 5 25:l;2

The reason for the high lixiviability of Wood impregnated with Composition XV (a) must be seen in the formation of the well-known, very stable but also easily water-soluble chromium-ammonia complex salts.

In Composition XV (b), the presence of both potassium and sodium causes the formation in the Wood of cryolites having the formulae K NaCrF or KNaCrF l-I O which are very diificultly soluble and therefore show a small lixiviability.

The formation of these cryolites is confirmed by the fact that in Equations M and N the total amount of fluorine added (bifluoride component and proton donator) may be reduced from 12 to 10 gram atoms of fluorine Without substantially changing the final results shown in Table 4.

The fact that the wood impregnated with Composition XV (b) contains, apart from the very difiicultly soluble cryolites, only volatile compounds (ammonia and Water), makes this composition particularly suited for impregnating railroad ties of beech, oak, pine and similar woods, whose natural electric conductivity should not be materially increased by impregnation, so that the electric signalling system may function unimpaired.

Thus a composition imparting to the wood an especially low electric conductivity is given in the following example:

EXAMPLE XVI To the composition described under VII, an ammonium arsenate is added to further decrease the electric conduetivity of wood impregnated therewith.

From a composition containing 5.0 g. KFHF 5.7 g. (NH AsO 3.2 g. CrO and 20 l. of water there is prepared after adding 2.6 g. H 1 (50%) a 1% solution with which small cubes of pine sap wood are vacuum impregnated. The sap wood cubes which are thus impregnated show after a storage period of 20 weeks an electric resistivity of'less than /5 compared withthe usual resistivity shown by wood treated with the known Wolman salt compositions under the same conditions. The Water content of the impregnated and unimpregnated timbers was during measurement between 26 and 30%.

EXAMPLE XVII A composition is prepared according to Example XIV (c) in which the sodium bisulfate is replaced by sodium pyrosulfate as an indirect proton donator.

The composition consists of 57.2 kg. KEHF 31.2 kg. Na Cr O ZH O 11.6 kg. Nagsgoq When dissolving the composition, NaI-ISQ; is formed from Na S O and acts as a proton donator.

EXAMPLE XVIII In order to show the efiect of using a'salt between a strong acid and a weak base, ammonium sulfate is used in the composition of-Example XVII, whichthen consists of 60.1 kg. KRHF 32.7 kg. NEZCIQO'IIZHZO 7.2 kg. (NH SO herein certain specific examples of the practice of this invention, it is not intended thereby to have this invention limited to or circumscribed by the specific details of materials, proportions or conditions herein specified, in view of the fact that this invention may be modified according to individual preference or conditions without necessarily departing from the spirit of this disclosure and the scope of the appended claims.

What I claim is:

1. A wood-preserving composition containing a fluoride, a chromate, and an arsenate in the proportions of 2/z moles of a chromate, 6 m/ moles of an alkali fluoride, it moles of an alkali arsenate, and a(6+bc)/d moles of an acid where z is the number of chromium atoms in the chromate molecule, 1 is the number of fluorine atoms in the fluoride molecule, m+n=2, a isany value from 0.25 to 1.2, b is the number of alkali atoms in the chromate molecule, c is the number of hydrogen atoms in the molecule of anhydrous arsenate, and d is the valence of the acid, the composition being substantially free from Cu, Zn, Ca and Mg compounds.

2. The composition of claim 1, in which the specified ingredients are respectively NaF, K Cr O and KH AsO 3. The composition of claim 1, wherein the acid is H 80 4. The composition of claim 1 containing a fluoride, a chromate, and an arsenate, and in which the fluoride is present in larger amount by weight than the arsenate.

5. The composition of claim 1 containing a fluoride, a chromate, an arsenate, and a bisulfate, and in which the fluoride is present in larger amount by weight than the arsenate.

6. A process for preserving wood in which the Wood is treated with the composition of claim 1.

7. A process for preserving wood in which the wood is treated with the composition of claim 2.

8 A process for preserving wood in which the wood is treated with the composition of claim 3.

9. A process for preserving wood in which the wood is treated with the composition of claim 4.

10. A process for preserving wood in which the wood is treated with the composition of claim 5.

References Cited in the file of this patent FOREIGN PATENTS 425,781 Great Britain Mar. 21, 1935 811,285 France Apr. 10, 1937 567,709 Great Britain Feb. 28, 1945 

1. A WOOD-PRESERVING COMPOSITION CONTAINING A FLUORIDE, A CHROMATE, AND AN ARSENATE IN THE PROPORTIONS OF 2/Z MOLES OF A CHROMATE, 6 M/F MOLES OF AN ALKALI FLUORIDE, N MOLES OF AN ALKALI ARSENATE, AND A(6+B-C)/D MOLES OF AN ACID WHERE Z IS THE NUMBER OF CHROMIUM ATOMS IN THE CHROMATE MOLECULE, F IS THE NUMBER OF FLUORINE ATOMS IN THE FLUORIDE MOLECULE, M+N=2, A IS ANY VALUE FROM 0.25 TO 1.2, B IS THE NUMBER OF ALKALI ATOMS IN THE CHROMATE MOLECULE, C IS THE NUMBER OF HYDROGEN ATOMS IN THE MOLECLE OF ANHYDROUS ARSENATE, AND D IS THE VALENCE OF THE ACID, THE COMPOSITION BEING SUBSTANTIALLY FREE FROM CU, ZN, CA AND MG COMPOUNDS. 